1. Field of the Invention
The present invention relates to an optical apparatus such as an exposure apparatus that is suitable for use in the production of a device such as a semiconductor device, and a method of manufacturing a device using the exposure apparatus.
2. Description of the Related Art
Various processes are performed on a substrate (e.g. a semiconductor wafer substrate or a glass substrate) in the production process of semiconductor devices such as ICs, LSIs, and liquid crystal panels. Among these processes, the exposure process for printing a pattern is an essential process in semiconductor manufacturing. Exposure apparatuses (such as a stepper or a scanner) are known for performing this process.
Resists applied to a wafer are broadly divided into a polymer film that is efficiently subjected to a chemical reaction by an ionization radiation (such as ultraviolet rays, X rays, or an electron beam) and a chemical amplification resist in which a catalyst (an acid) is produced by exposure and an image is formed by the catalyst in post-exposure baking (PEB). The sensitively of such a chemical amplification resist can be easily improved because the image is formed using the catalyst. Therefore, recently, chemical amplification resists are generally used as a resist for excimer laser beams, by which it is difficult to provide a satisfactory illuminance.
On the other hand, in the chemical amplification resist, the catalyst generated by exposure diffuses in air and on the surface of the wafer and the catalytic action is accelerated by the post-exposure baking (PEB), thereby degrading the image profile. Accordingly, in use of the chemical amplification resist, the control of chemical pollution due to a basic gas such as an amine or an amide is required in the environmental atmosphere during the steps of resist application, exposure, and post-exposure baking (PEB).
An exposure apparatus includes various optical members such as lenses and mirrors, in addition to an illumination optical system that irradiates light emitted from a light source on the surface of a reticle (original plate). As the exposure wavelength decreases, the optical members that transmit the exposure light and that are irradiated with the exposure light become clouded, resulting in a decrease in the light exposure reaching the surface of the wafer. The substances that cause the clouding are organic compounds and ammonium sulfate ((NH4)2SO4). The possible cause is that ammonium ion (NH4+), sulfate ion (SO42−), a compound thereof, or an organic gas that is contained in air is subjected to a photochemical reaction by irradiation of the exposure light, and the resulting product adheres to the optical members.
Hitherto, in order to overcome problems that the surface of the chemical amplification resist becomes insoluble and the optical members become clouded, an impurity-removing filter is installed in an environmental chamber that controls the temperature, the humidity, and dust in the environment surrounding the main body of the exposure apparatus to remove substances such as a basic gas, a sulfuric acid gas, and an organic compound gas in the atmosphere.
As additional countermeasures, components used in the environmental chamber are subjected to treatments such as washing, and a lubricant or the like that generates a small amount of outgassing of the above components is selected for use. Also, a part of or all of the space surrounding the optical path from a light source to a substrate to be processed is purged with a gas inert to the exposure light.
Japanese Patent Laid-Open No. 2001-028331 discloses an example of purging with an inert gas. The invention described in this patent document focuses on a method of supplying a gas for purging when a casing of an optical apparatus is purged with an inert gas, i.e., nitrogen, and aims to reduce the contamination of optical members such as lenses due to the gas to be supplied.
When the gas supplied from a gas supply unit at a predetermined flow rate is introduced in the casing, the flow rate of the gas is decreased relative to the predetermined flow rate. Consequently, when the gas reaches the surfaces of the optical members, the flow rate of the gas is decreased and the gas is easily dispersed, and thus impurities contained in the gas are not easily adhered to the optical members.
Furthermore, a space from a light source to a leading end of a projection optical system has a hermetically sealed structure to separate from the outside air. This structure prevents impurities from adhering to the surface of an optical member of the optical apparatus, the surface not being in contact with the outside air.
According to the invention described in the patent document, a space from the light source to the leading end of the projection optical system or to a wafer stage space has a hermetically sealed structure to separate it from the outside air, thereby preventing impurities from adhering on the optical members.
However, when the space from the light source to the leading end of the projection optical system or to the wafer stage space has a hermetically sealed structure, the apparatus is increased in size, and an opening and closing mechanism is essentially provided at positions for carrying in and out a mask (original plate) and a wafer (substrate). Therefore, in order to maintain the purging performance, the operation ratio of the apparatus is disadvantageously decreased.
In addition, since almost all parts of the apparatus are purged, a large amount of inert gas is required, resulting in an increase in the operating cost of the apparatus.
In the invention described in the above patent document, impurities contained in the gas for purging are not easily and intensively adhered on the surfaces of the optical members in the hermetically sealed casing, thus achieving the main purpose of the invention. However, when the impurities or a space containing the impurities is irradiated with exposure light to generate a substance that adhere on the surfaces of the optical members by a photochemical reaction, the above invention does not provide an effect of suppressing the amount of the substance that adhere as long as the concentration of the impurities is constant and the supply flow volume of the purge gas is constant. Therefore, the clouding of the surfaces of the optical members cannot be prevented.
Furthermore, although the intensive adhesion may be prevented, the impurities may be adhered on the other parts. When the impurities are partially and intensively adhered, it is effective that such a part is disposed away from the area used for the optical system. On the other hand, in the structure described in the patent document, the possibility of adhesion of the impurities is generated in all the optical members provided on the purging pathway. As a result, the transmittance of the optical members is disadvantageously decreased.
In order to prevent the size of the apparatus from increasing, the optical apparatus has a hermetically sealed structure and the inside thereof is purged, and the clouding of the optical members of the optical apparatus being in contact with the outside air is prevented with an impurity-removing filter provided in the environmental chamber. Although this structure is realistic, even when the impurity-removing filter is provided in the environmental chamber, the impurities cannot be completely removed.
In addition, such an impurity-removing filter has a lifetime for maintaining the removing performance depending on the amount of impurities removed, resulting in problems of maintenance and operating cost of the filter.